As you do missions for NPC corporations, your standings with them will go up. As you do storyline missions, your faction standings will change for the faction a particular NPC corporation is part of.

Gaining faction standings via storyline missions or data center missions carries "Derived Modifications", meaning that standings will also go up with friends of that faction and down with enemies of that faction. For example, raising your faction standings with the Gallente Federation will also raise it slightly with the Minmatar Republic while lowering it slightly with the Caldari State and Amarr Empire. The reverse also being true - raise Amarr, lose some Gallente faction standings. (Epic Arc mission chains do not carry derived modifications; they only affect the standings of one faction, depending on mission choices made during the epic arc.)

These changes are where some of the social skill effects come into play. Others are specific to mission runners.

Increasing the Charisma and Intelligence attribute scores increases the rate at which Social category skills are trained.

The social skills affect 3 different areas of your interactions with NPC entities (meaning Factions, Corporations, and Agents):

Connections - 4% effective standings increase for non-criminal NPC entities that you have positive standings with. (Note that the in-game description for this skill neglects to mention that it doesn't apply to criminal entities.)

In the above, "criminal" refers to the NPC pirate factions, or the member corporations and agents thereof. The NPC pirate factions have negative standings from (and towards) CONCORD. Diplomacy, Connections, and Criminal Connections do NOT have overlapping validity conditions; only one of the three will apply.

Loyalty Point Reward ImprovementsLoyalty Point (LP) reward improvements are broken down by corporate divisions. All are 10% gains, per level trained, in LP rewards while working for an agent from their specific division.

Diplomacy

Skill at interacting with hostile Agents in order to de-escalate tense situations as demonstrated by some of the finest diplomats in New Eden. 4% Modifier per level to effective standing towards hostile Agents. Not cumulative with Connections or Criminal Connections.